Nickel-base alloys



United States Patent 3,174,851 NICKEL-BASE ALLOYS William J. Buehler,Hyattsville, and Raymond C. Wiley,

Rockville, Md., assignors to the United States of America as representedby the Secretary of the Navy No Drawing. Filed Dec. 1, 1961, Ser. No.157,049

3 Claims. (Cl. 75-170) (Granted under Title 35, US. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for Governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to a series of novel structural alloys of theintermetallic compound type which are characterized by unusualmechanical and physical properties.

Most intermetallic compounds, other than TiNi, are very brittle, lackany form of ductility at room temperature and in spite of many otheroutstandingly good properties displayed by these compounds, such asstrength maintenance at high temperatures, their brittleness at roomtemperature has made these compounds virtually useless in structuralapplications except as minor strengthening constituents in a moreductile matrix metal or alloy.

Novel intermetallic compound base materials of the TiNi type have nowbeen discovered which not only possess the desirable propertiescharacteristic of intermetallic compounds in general but also possesshitherto unknown and unusual properties.

Accordingly, it is an object of the present invention to provide a newseries of structural alloys of the intermetallic compound typecharacterized by high strength at room temperature and at elevatedtemperatures, good oxidation resistance up to a moderate fraction of themelting temperature, good corrosion resistance, moderate density,reasonable ductility and impact resistance at all temperatures, and goodweldability and being further characterized by non'magnetic stability atuseful temperatures and unusual mechanical vibration damping propertieswhich are sensitive to both composition and temperature changes.

It is a further object to provide intermetallic compoundbase alloyscapable of being readily melted, cast into a chemically homogeneoussolid mass and worked hot (above recrystallizationtemperature), cold(below recrystallization temperature) or by hot and cold means to afinal usable shape.

It is yet another object to provide novel intermetallic compound alloyscapable of heat treatment to any required hardness value fromapproximately about 65 R to approximately about 62 R It is a stillfurther object to provide a non-magnetic alloy capable of being heattreated or developed to high hardness and strength for use innon-magnetic tools and other functions in connection with magneticsensitive devices.

The term intermetallic compounds as a general term is consideredhereinafter as an intermediate phase in an alloy system, having areasonable range of homogeneity and relatively simple stoichiometricproportions, in which the nature of the atomic binding can vary frommetallic to ionic, and further includes all intermediate phases inbinary and higher order metal systems whether ordered or disordered.These intermetallic compounds are combinations of two or more metals,the atoms of such metals generally being in a simple whole number ratio.In the majority of cases, however, the formulas of intermetalliccompounds do not agree with formulas based on the principle of valency.

The novel alloys developed in accordance with the present inventionoccur in three possible phases as illustrated by the followingequilibrium equation:

TiNi=Ti Ni+TiNi They were prepared as illustrated in the followingdetailed example.

EXAMPLE The raw materials used were Mond nickel shot and commerciallypure titanium bar stock.

The preparation of the novel intermetallic TiNi alloys may be dividedinto three distinct steps as follows:

(a) Melting of the alloy (b) Working of the arc-cast alloys (c) Heattreating the wrought materials Melting of the alloys The alloys, due totheir high titanium content, may be melted by either consumable ornon-consumable are methods or the like employing a Water-cooled coppercrucible or hearth.

Working of the arc-cast alloys All of the cast alloys between about 52to 56 weight percent nickel and correspondingly between about 48 to 44Weight percent titanium may be hot worked without any preliminary heattreatment. A stoichiometric TiNi composition of 55.1 weight percentnickel and 44.9 weight percent titanium was readily hot worked in the ascast condition between about 650 C. to about 1100 C., the preferred hotworking temperature range being from about 700 C. to 900 C.

Alloys containing above 56 weight percent nickel, that is from about 56to 64 weight percent nickel required a preliminary heat treatment torender them hot workable. This heat treatment consisted of heating thealloys to about 1050 C. until heated through and then slowly cooling toroom temperature. Alternatively, the alloys may be heated to about 1050C. until heated through, cooled slowly to about 850 C. and held at thattemperature until heated through and subsequently allowed to slowly coolto room temperature. The principle behind the above pre-working heattreatments is to precipitate and coalesce the excess compound phaseTiNi3 from solution with the compound phase of TiNi. This results in aductile TiNi matrix interspersed with the more brittle TiNi compoundcoalesced into harmless particles. Following the above describedalternative pre-working heat treatments the two phase TiNi-l-excess TiNialloy was capable of being rolled at any temperature between about 700C. to about 900 C.

35 Heat treating the wrought materials The hardness of alloys containingbetween about 52 to 56 weight percent nickel (remainder titanium) andbeing predominantly single phase TiNi was only very slightly affected byany heat treatment regardless of the rate of cooling.

Conversely, the hardness of alloys containing between about 56 to 64weight percent nickel (remainder titanium) are very much affected byheat treatment and particularly by the cooling rate. These alloys whenheated to above about 900 C. and quenched in room temperature water.attain a high hardness. For instance, an alloy of about 60 weightpercent nickel and about 40 weight percent titanium yielded, whenquenched from between 900 C. and 1110 C., a hardness varying between 58R and 62 R as may be seen from Table I.

Table 1 AVERAGE HARDNESS OF 60 Ni-40 Ti (WEIGHT PER- CENT) ALLOY WATERQUENCHED FROM DIFFERENT TEMPERATURES Quenching Temp, C. Harilincss,Remarks 1,110" O 62 Heat treated in an air atmosphere. 1,000 C V 01 Do.900 C 58 Do.

The same 60 Ni-40 Ti (wt. percent) alloy when furnace cooled (averagecooling rate about 50" C./hr.) attained a final hardness of about 35 Rsuch hardness being approximately equal to the hardness of alloycompositions being in the T iNi phase (52 to 56 weight percent nickel)as shown in Table II.

The above data clearly show the capability of the nonstoichiometric TiNialloys (i.e., those containing excess nickel) to be hardened byquenching. It is also clear that an alloy of 56 wt. percent Ni(remainder Ti) is the transition between hardenable and non-hardenablealloys and that a great excess of the hardening constituent TiNi (aboveabout 64 Wt. percent nickel) serves to reduce the quenched hardness.Thus the preferred range, for maximum hardness, would be between 58 and62 wt. percent Ni.

The hardness of quench-hardened alloy (56 to 64 wt. percent Ni,remainder Ti) components may be reduced to a lesser degree of hardnessif such is required for a specific application. Such reduction must bebased upon the best compromise of mechanical properties for theparticular application. Reduction of hardness of quenchhardened all-oysmay be accomplished by (A) slowing down the cooling rate of the heattreated component to yield hardnesses between about 35 R (furnace cool)and about 62 R (water quench) and by (B) tempering. This temperingprocess is accomplished by reheating the quench-hardened alloy tovarious temperatures below the point of change in slope of the phaseboundary between the TiNi and TiNi-i-TiNi phase areas (about 900 C.) andcooling at a specified slow rate, the final tempered hardness beingdetermined by the heating temperature, period of time at the heatingtemperature and the rate of cooling. In order to minimize surfaceoxidation (when heating above about 600 C.) the above hardening andtempering heat treatments may be performed in a controlled atmosphere ofhelium or argon. In many applications, however, heat treatment in airwill sufiice.

Further hardness data for TiNi, Ti Ni and TiNi compositions are shown inTable III.

Table III Alloy Alloy Melting Hot Roll Temp, C Hardness, Composition RTiN i Non-consu1nable 30-31 TiNi TiNi TiNi 'liNL As cast (No H.R.)..

From the above data it may be seen that the room temperature hardnessincreases with an increase in rolling temperature which is undoubtedlyrelated to the higher temperature of heatingand fairly rapid coolingrate from temperature. It may also be seen that while the Ti Nicomposition alloy is quite hard (53 R the TiNi compound has a hardness(34 R more like that of the TiNi alloy. Yet in spite of the much lowerhardnessexhibited by TiNi it is similar to the Ti Ni compound in that itis brittle even at high homologous temperatures.

A particularly unusual property was observed of these novel alloyscontaining from about 50 to 70 wt. percent Ni (remainder Ti) and thiswas the property of these alloys to retain their hardnesscharacteristics independent of temperature, for example, at temperaturesranging from about room temperature up to about 463 C. and down to aboutC. Alloys characterizedby an essentially pure TiNi phase (54.5 to 55.1W/o Ni-Ti have even shown a tendency to exhibit a secondary hardening.This is indicated in Table III, column 8 Tensile properties weremeasured on both 54.5 and 55.1 wt. percent Ni alloys (remainder Ti). Inevery case, a standard specimen measuring 0.252 diameter x 1.0" gagelength was employed. The test sections were finish lapped in thelongitudinal direction to avoid any possible transverse notches. Toavoid oxidation of the prepared sample surfaces and minimize thepossible interstitial element (0, N, H) pickup, vacuum or controlledatmosphere heat treating was used. Vacuum heat'treating was performed inan evacuated quartz tube. The tensile test results obtained from the twoTiNi alloys are shown in Table IV.

Table IV TENSILE TEST DATA 54.5 w/o N i-Ti Alloy (room temperature)Ultimate Yield Elonga- Reduction Modulus of Specimen Treatment TensileStr., Strength b tion, in area, Elasticity, Remarks s linfl lbs/in.percent percent p.s.i.

800 C. 1 hour furnace cooled e 112, 100 40, 000 8. 1 11. 6X10 Bgokegutside gage en t 800 C. 1 hour water quench e 123,800 40, 700 15. 16.011. 8X10" g 54.5 w/o Ni-Ti Alloy (185 to 192 F. test temperature 800 C.1 hour furnace cooled e 110,500 46, 800 3. 6 11. 1X10 Biioke olutsidegage engt 800 C. 1 hour water quench e 115, 300 55,100 10. 9 13.0 11.2X10 55.1 W/O Ni-Ti Alloy (room temperature) Hot swaged at 900 0., aircooled 125, 000 81, 400 8.1 1,000 C. min., furnace cooled 116, 700 56,200 7. 2 Hard-:26 R

Do .1 114, 200 33, 600 3. 2 Broke outside gage lRength. Hard=24 Des 140,500 as, 200 9.9 Hard =24 3.. 1,000 C. 30 min, water quench 82, 320 71,400 3. 5 Hard=33 Rt.

Do. 84, 400 62, 250 4. 5 l1. 8X10. Hard=31 Be.

* Tensile specimen size 0.252 dia. x 1.000 gage length.

b Offset- 0.2%.

e Heat treatment performed in an argon atmosphere.

6 Specimen sealed in an evacuated quartz tube during heat treatment.

Upon observation of the above data it becomes appar- Table VI ent thatthe ductility, as indicated by the percentage 30 elongated, can go asbig has 15.5% with the average being in the 7-10 range. For anintermetallic compound this is an unusually and unexpectedly high roomtemperature elongation. Moreover, it is seen that the yield CORROSIONCHARACTERISTICS Corrosive media: Resultant attack Salt spray, 20% soln.,

95 F. for 96 hrs. Faint whitish surface destrength varies considerablywith composition and heat 35 treatment while the ultimate tensilestrength and modulus posit on b k edge, f of elasticity are fairlyconstant regardless of composition specimm NO attack on and heattreatment. 6X osed surface For determination of impact properties,carefully p prepared unnotched square cross-section bars were used. 4Sea Water 192 The specimen surfaces were hand lapped in the longitudi-Normal air atmosnal direction to minimize transverse scratches. Thephepe Nil. Charpy impact tests were performed in a standard RiehleNormal handhn N11. machine. The resulting data are summanzed in Table V.g

Table V IMPACT DATA FOR TiNi COMPOSITION ALLOYS 1 GIVEN PRIOR THERMALTREATMENTS AND AT VARIOUS TEST TEMPERATURES Nominal Alloy SpecimenSection Conditions of Test Charpy Composition Size Impact ft.lbs.

54.5 w/o Ni-Ti 2 0.296 x 0.296 Test Temperature: 75 F. (R.T.) 28 54.5w/o Ni-Ti 0.296 x 0.296 Test Temperature: 125 F 32 54.5 w/o Ni-Ti 0.296x 0.296. Test Temperature: 200 F- 29. 5 54.5 w/o Ni-Ti 0.296 x 0.296.Test Temperature: 112 F 40 54.5 w/o Ni-Ti 2 0.206" x 0.206 Cooled to l12F., Warmed in RT. water, 23

stabilized 15 min. in R.T. air. 54.5 w/o Ni-Ti 2 0.296 x 0.296 Cooled to112 F., Warmed in RT. water, 25

stabilized 15 min. in RT. air, plus heat to a test temperature of 160 F.55.1 w/o Ni-Ti 3 0.297 x 0.297" Test Temperature: F. (R.T.) 24 55.1 w/oNiTi 0.297 x 0.297 Test Temperature: 200 F 8 55.1 w/o Ni-Ti 3 0.207 x0.297 Test Temperature: -l12 F- 43 1 Unnotched square cross-section barswere employed. 2 Specimens prepared from hot swaged (0000 0.) bars. 3Specimens prepared from hot rolled (900 0.) plate. Again, as in the caseof the tensile elongation, un- Table VII usually high impact strengthswere attained as compared OXIDATION orrannc'rnnrs'rros with mostintermetallic compounds. For both of the TiNi alloys the minimum valuewas 23 ft.-lbs. even on the undersize specimens. Especially to be notedis the Weight Gain (Grns.) due to Oxidation of Various TemperaturesTesting Time, Hrs.

increase in impact strength at temperatures well below 800C, 1 0()[)(]freezing.

Specimens of a 55.1wt. percent Ni (remainder Ti) alloy .016 .007 wereexposed to various common corrosive media and 1853 to elevatedtemperature oxidation attack. The results of 3 these tests aresummarized, respectively, in following 1 5 I Tables VI and VII.

It will be noted from Table VI that in each case the attack wasnegligible and only in the rather drastic salt spray tests was aperceptible film formed where the specimen was held. The passivity ofthis alloy to corrosive attack is obviously a highly desirablecharacteristic.

From Table VII it will be seen that at 600 C. there was very littleinitial oxidation and oxide buildup was almost negligible after thefirst two hours. At 800 C. oxidation proceeded steadily after the firsttwo hours and at 1000" oxidation was initially rapid and proceededsteadily. At 800 C. and 1000 C. spalling of the oxide surface wasmoderate to heavy.

In the fabrication of present day structural materials joining is anextremely important consideration. To obtain an indication of theWeldability of the TiNi material, two chamfered A5" thick hot rolledplates of TiNi were butt welded together by the heliarc method. Littleditliculty was encountered in making the joint and the weld sectionappeared to be free of cracks and porosity. Based upon the observedproperties in this arc-cast TiNi material, the weld section should bequite strong and tough. Further, examination of the magnetic propertiesof the weld section indicate that it is equally as paramagnetic as thebase material.

Among the various unusual properties exhibited by these novel alloys,the property of paramagnetism is of utmost importance. A paramagneticmaterial has been defined as a material whose specific permeability isgreater than unity and is practically independent of the magnetizingforce. The nickel-titanium alloys in the composition range which coversTi Ni, TiNi, and TiNi are highly paramagnetic, in spite of the highamount of nickel present in there alloys. Alloys of the 54 to 60 w/o Ni,remainder Ti composition have been magnetically evaluated after variousthermal treatments and at widely varying temperature. The magnetictesting included both magnetic susceptibility and permeabilitymeasurements. In these tests it was found that the permeabilityapproached extremely close to unity regardless of the temperature,residual stresses, or prior thermal treatment. Care must be exercised toremove any oxide coating in cases where the TiNi-base alloys are to beused in non-magnetic applications. This is caused by the combination ofsome Ni of the base alloy with O to form a ferromagnetic oxide coating.

Of considerable importance in regard to these novel alloys is theunusual characteristics exhibited by the mechanical vibration dampingeifect. The stoichiometric alloy in both the arc-cast and hot workedconditions exhibits a unique and drastic change in vibration dampingwith minor changes in temperature and composition. Quantitative andqualitative experiments have shown the damping of a 54.5 w/o Ni-Ti alloyWith minor amounts of Fe (about 0.1 w/o) to change from a highly dampingmaterial at room temperature (25 C.) to-a very low vibration dampingmaterial at 54 C. and above. Internal friction experiments performed on0.036 diameter wire showed the logarithm of amplitude to decrease from2.3 to 1.1 in 35 cycles at room temperature (25 C.) while the same wiredrops from 2.3 to 2.1 in 35 cycles when heated at 93 C. This illustratesclearly the damping change in wrought wire of the 54.5 W/o Ni,approximately 0.1 w/o Fe, remainder essentially Ti. Even more'drasticchanges in damping behavior are exhibited by this composition alloy whenin the arc-cast condition. These changes in damping appear to beassociated with the phase equilibria of the alloy system. As thetemperature is raised the TiNi phase increases in quantity at theexpense of reducing the extraneous phases present (Ti Ni and TiNi Asthis occurs damping is markedly 'decreased.

The phase equilibria theory is further confirmed by the fact that alloyscontaining excess Ni or excess Ti over the stoichiometric compositionhave distinctly different room temperature damping properties. Forexample the Tib cs rich alloys (less than 54.5 w/o Ni) are highlydamping at room temperature, while alloys on the Ni-rich side (in excessof 54.5 w/o Ni) show low damping at room temperature, thus indicatingthat the Ti Ni phase coupled with the liNi produces the high dampingcapacity. Anything lessening the Ti Ni phase, e.g. increased Ni, highertemperatures, impurity atoms like Fe, etc. causes minor changes in theTiNi/Ti Ni phase equilibria and thus promotes drastic vibrationaldamping changes- This unusual damping phenomenon may lead to theutilization of these alloys as temperature sensing devices.

A summary of the properties of the novel TiNi base alloys is presentedin Table VH1. (See column 8.)

In summary, novel TiNi alloys containing from about 50-70 wt. percent Ni(remainder Ti) have been discovered which possess the unusualcombination of properties of high hardness at wide temperaturevariations and especially at temperatures Well below freezing and havingunusually good ductility. and impact strength at these sametemperatures. Within this range of 50-70 wt. percent Ni, the alloys maybe subdivided into those alloys having a range of about 52 to about 56wt. percent Ni (remainder Ti) and those alloys containing from about 56to about 64 wt. percent Ni (remainder Ti). The former are characterizedby the existence of an almost wholly TiNi phase, by being readilyworkable whether hot or at room temperature and by exhibition ofunusually high ductility at room temperature. The latter alloys arecharacterized by being two-phase materials (TiNi-I-TiNig) capable ofbeing hardened to'high hardness levels. The combination of the highhardness of these latter alloys and their characteristic paramagnetism(magnetic permeability=less than 1.02) is of great importance and leadsto their use as superior non-magnetic tools magnetometer applicationsand structural materials which will remain stably nonmagnetic, free ofcorrosive attack, and resistant to abrasion.

Table VIII SUMMARY OF PROPERTIES OF TiNi PHASE ALLOYS [Physical (55.1w/o Ni-Ti)] Density (25 C.), gr./cm. 6.45.

Melting point, C. 1240-1310. Melting point, F. 2264-2390. Crystalstructure CsCl (B.C.C.). Lattice parameter, A 3.015.

Electrical resistivity (25 C.), microhm- Electrical resistivity (900C.), mi-

crohm-cm. -132.

Linear coef. of expansion (24-900 C.),

per C. 10.4 10- Recrystallization temperature, C. 550-650. Magneticpermeability l.002.

Magnetic susceptibility (mass, -196 to 550 C.) 5-9 10' [Mechanical] 54.5w/o Ni V 55.1 w/o Ni Ultimate Tensile Stn, p.s.i-..- 110,000-124,000-82,000-140,000.

Yield Str., p.s.i** 40,G00-55,000 33,00081,400.

Youngs Modulus, p.s.i 11.2-11.8 X 10 Up to 11.8 X 10 Tensile Elongation,Up to 15.5- Up to 10.

percent? Reduction in Area, psrcent-... Up to 1G Hardness. Rockwell-A42-52 65-68.

Hot Hardness," D.P.H.:

Impact Str., ft.-lbs.:

24 0. (room temp.) C Modulus of Rupture, p.s.i.. Mod. of Elas. (Trans.Bend),

p.s.i.

25 0 (room temp.) 0 (3....

*Specimeus rapidly cooled prior to testing. Percent ollset=0 2%. Gage1ength=1 1H.

It should be understood, of course, that the foregoing disclosurerelates only to preferred embodiments of the novel alloys of theinvention and that modifications or alterations may be made thereinwithout departing from the spirit and scope of the invention as setforth in the appended claims.

Having thus described the invention, what is claimed and desired to besecured by Letters Patent of the United States of America is:

1.A novel alloy composition consisting essentially of from about 50percent to about 70 percent nickel by weight and correspondingly fromabout 50 percent to about 30 percent titanium by weight, said alloyhaving the structure of a TiNi phase in combination with a TiNi pha-sedispersed in a TiNi matrix within a temperature range of from about 500C. to about -75 C. and having the properties of being paramagnetic, ofretaining hardness throughout said temperature range and of beingcorrosion resistant.

2.A novel alloy composition consisting essentially of from 52 percent toabout 5 6 percent nickel by weight and correspondingly from about 48percent to about 44 percent titanium by weight, said alloy having thestructure of a substantially TiNi phase within a temperature range offrom about 500 C. to about 75 C. and having the properties of beingparamagnetic, of being highly vibration damping at about roomtemperature and having the capability of being plastically deformed atabout room 1t) temperature and retaining the deformed shape until heatedto a higher temperature whereupon the prior nondeformed condition willbe reassumed.

3. A novel alloy composition consisting essentially of from about 56percent to about 64 percent nickel by weight and correspondingly fromabout 44 percent to about 36 percent titanium by weight, said alloyhaving the structure of a substantially TiNi phase dispersed in a TiNimatrix within a temperature range of from about 450 C. to about 75 C.and having the properties of being paramagnetic, of high hardness uponheat treatment, of being abrasion and corrosion resistant and capable ofbeing hot wrought into useable shapes prior to hardening.

References Cited by the Examiner I. J. Wallbaurn, Archiv. Fiir DasEisenhuttenwesen, JG 14, 1940-1941, pages 521-526.

Hansen: Constitution of Binary Alloys, published by McGraw-Hill BookCo., Inc., New York, 1958, pages 1049-1053.

Poole et al.: The Equilibrium Diagram of The System Nickel-Titanium,Journal of the Institute of Metals, vol. 83, pages 473-480, July 1955.

DAVID RECK, Primary Examiner.

RAY K. WINDHAM, WINSTON A. DOUGLAS,

Examiners.

1. A NOVEL ALLOY COMPOSITION CONSISTING ESSENTAILLY OF FROM ABOUT 50PERCENT TO ABOUT 70 PERCENT NICKEL BY WEIGHT AND CORRESPONDINGLY FROMABOUT 50 PERCENT TO ABOUT 30 PERCENT ITANIUM BY WEIGHT, SAID ALLOYHAVING THE STRUCTURE OF A TINI PHASE IN COMBINATION WITH A TINI3 PHASEDISPERSED INA TINI MATRIX WITHIN A TEMPERATURE RANGE OF FROM ABOUT500*C. TO ABOUT -75*C. AND HAVING THE PROPERTIES OF BEING PARAMAGNETIC,OF RETAINING HARDNESS THROUGHOUT SAID TEMPERATURE RANGE AND OF BEINGCORROSION RESISTANT.